Thread: Typedef pointer array access question

Originally Posted by fxtdr79

I have a data structure declared like this:

Code:

typedef struct {
   double x;
   double y;
} xyPoints;

Then I created a pointer like this:

Code:

xyPoints *points[30];

I can't seem to store anything in the x and y values. I'm try to do something like this:

Code:

points[0].x = 0.0;

No matter what combinations of * and & I try with points[0] I keep getting an error message when I try to compile. What is the proper syntax to gain access to the x and y values at a specific point in the array. Thanks.

Hi Please post your complete code. I had a similar problem a few months ago (I'm a C student also) and found that I was initializing my pointers in the wrong place.

I don't know if that is the problem here, but I did learn one thing: it's important to post
your entire code. :-)

You need to use -> instead of . to access the members.

Originally Posted by claudiu

You need to use -> instead of . to access the members.

Yes, but I believe there are 3 ways to access the members.

In my book, I read "If p_str is a pointer to the structure str, the following 3 expressions are
all equivalent:

1) str.memb

2) (*p_str).memb

3) p_str -> memb

In other words, there are 3 ways to access a structure member: using the structure name,
or using a pointer to the structure with the indirection operator (*) or using a pointer to
the structure with the indirect membership operator (-> )

I wasn't clear on the problem, as I had a similar problem a few months ago. It turned
out to be where I initialized my pointer, and Elysia set me straight on that one. :-)

Sure, there are three equivalent ways. However, regardless of which method you use, it is necessary to use one that is correct.

Your code snippet creates an array of 30 pointers, not a pointer. Those 30 pointers are not initialised so, if you actually want to do anything useful, they need to be initialised (i.e. made to point at valid xyPoint objects)

Once they are initialised, you need to use appropriate syntax to access members of the individual structs.

For example;

Code:

xyPoints *points[30];
for (int i = 0; i < 30; ++i) points[i] = malloc(sizeof(xyPoints);
points[0]->x = 42;
/*  cleanup  */
for (int i = 0; i < 30; ++i) free(points[i]);

More simply, create an array of 30 xyPoints

Code:

xyPoints points[30];    /* an array of xyPoint's, not an array of pointers */
points[0].x = 42;

Your basic problem seems to be that you are misunderstanding the notions of pointers and arrays. A pointer is a different thing from an array. Your code is creating an array of 30 pointers contrary to your description as begin "creating a pointer". All 30 of those pointers are uninitialised unless YOU initialise them.